There is a need for a cement which has the setting and volume change characteristics normal for portland cements but which has early very high compressive strengths. This need is particularly acute for portland cements used in the prestress and precast concrete industry. Considerable savings result from the required strength being obtained at early ages so that construction can continue and there is a more rapid reuse of molds. There is also a need for such a very early high-strength cement having the setting characteristics of portland cement in patching or resurfacing highways and airport runways or in any operation where early form removal is desired.
While there have been proposals in the past for a cement having early high compressive strength, none of them have had the early compressive strengths required without the use of chlorides or other accelerators.
The following United States of America patents are directed to manufacture of cement involving separation of cement particles as well as various separation processes directed to various materials: 1,592,343; 1,943,817; 2,214,715; 2,214,716; 2,288,179; 2,424,746; 3,013,786; and 3,638,400.
Of the foregoing patents, Downes (U.S. Pat. No. 2,424,746) discloses a process of manufacture of cement in which cement is carried through a centrifugal separation process to separate cement particles of a size smaller than 100-mesh while those of a larger size are recycled to the tube mill.
The patent to Pyzel (U.S. Pat. No. 3,013,786) is directed to a hydraulic cement process in which the cement is subjected to a separating step in which particles of 200 micron sizes are separated from particles of smaller size.
The patent of Heilmann (U.S. Pat. No. 3,638,400) refers to a separation of alkali-rich dust having a particle size larger than 5 to 15 microns from cement by electrostatic techniques for the purpose of controlling the alkali content of cement clinker.
The remaining patents are generally directed to separations of finer particles from coarse ones by various techniques.
None of the foregoing patents, however, discloses or suggests a cement having substantially all of its particles of about 20 microns and smaller by which unexpected and highly advantageous early high-strength properties are obtained; that is, cement having a compressive strength better than 1,000 psi by 6 hours and 6,000 psi by 24 hours when tested in a standard 1 to 2.75 by weight cement-sand mortar, having the setting and volume change characteristics that are normal for and are typical of portland cement, and having longer plasticity than standard portland cement.